Chapter 4 PDF: Epithelial Tissues

Chapter 4: Epithelia line cavities of organs and cover the body surface with 3 main functions: o Secretion (glands) o Absorption (intestinal lining) o Covering, lining and protecting surfaces (epidermis) Can be of 3 sizes and shapes: o Columnar o Cuboidal o Squamous Most epithelia rest on connective...

Chapter 4: Epithelia line cavities of organs and cover the body surface with 3 main functions: o Secretion (glands) o Absorption (intestinal lining) o Covering, lining and protecting surfaces (epidermis) Can be of 3 sizes and shapes: o Columnar o Cuboidal o Squamous Most epithelia rest on connective tissue that contains microvasculature bringing nutrients and O2. Lamina propria: connective tissue that underlies the epithelia lining the organs in digestive, respiratory and urinary system. Papillae: evaginations between epithelium and connective tissue to reduce friction (skin or tongue) Basal pole: region of cell contacting the connective tissue Apical pole: face a space Basement membrane: specialized fatlike sheet of extracellular material at the basal surfaces. o 2 layers: ▪ Basal lamina: fine fibrils near the epithelial basal pole ▪ Reticular lamina: more diffuse and fibrous layer beneath the basal lamina. o Basal lamina secretes the following macromolecules into its ECM at the basal pole: ▪ Laminin: large glycoproteins that self-assemble. ▪ Type 4 collagen ▪ The laminin and type 4 collagen are held together by adhesive glycoprotein entactin/nidogen and by perlecan. o Reticular lamina contains: ▪ Type 3 collagen which is bound by type 7 collagen to basal laminae. o Basement membrane functions: ▪ Provide structural support ▪ Provide polarity to epithelial cells ▪ Attach epithelia to connective tissue Intercellular adhesion and junctions: o Tight junctions (Zonulae occludens): ▪ Most apical of junctions ▪ Cause by interactions between claudin and occludin of each cell. ▪ Makes sure molecules cross through transcellular path and not paracellular path (between 2 cells) o Adherent junctions (Zonula adherens): ▪ Immediately below tight junctions ▪ Firmly anchoring a cell to its neighbors ▪ Mediated by cadherins that interact with each other in presence of Ca2+ ▪ ▪ Cadherins bind to catenin on cytoplasmic side Desmosome (macula adherens): Doesn’t form a belt around cell, just at one spot. Disc shaped Contain desmoglein and desmocollin (members of cadherin family) Plakoglobin and desmoplakin are anchoring proteins on cytoplasmic side. o Gap Junctions: ▪ Mediate communication instead of adhesion or occlusion. ▪ Proteins called connexins form hexameric complexes called connexons whith hydrophilic core. ▪ Permit intercellular molecule exchange. o Hemidesmosomes: attach epithelial cells to subjacent basal lamina. They contain integrins instead of cadherins (unlike desmosomes) Microvilli: o Each microvillus contains many bundles of actin filaments. Stereocilia: o Absorptive epithelial cells lining the epididymis and the ductus deferens. o Are also important sensory cells of inner ear. Cilia: o Larger than microvilli o Core structure consisting of 9 peripheral microtubular doublets and 2 cenrtral microtubules. o The 9+2 assembly is called the axoneme which are continuous with basal bodies (apical cytoplasmic structures just below the cell membrane) o Movement of cilia is due to changes in axoneme. Types of Epithelia: o 2 main groups: Covering (lining) and secretory (glandular) o Covering epithelia: ▪ Simple epithelia: one cell layer ▪ Stratified epithelia: 2 or more layers. ▪ Squamous: thin cells ▪ Cuboidal: cell width and thickness are similar ▪ Columnar: cells taller than they are wide ▪ Stratified squamous keratinized epithelium: found in epidermis of skin where it helps prevent dehydration from the tisse. ▪ Stratified squamous nonkeratinized epithelium: lines wet cavities like mouth, esophagus, and vagina where water loss is not a problem. ▪ Stratified cuboidal and stratified columnar epithelial are both very rare. ▪ Stratified cuboidal: in excretory ducts of salivary and sweat glands. ▪ Stratified columnar epithelium: conjunctiva lining the eyelids ▪ Transitional epithelium (urothelium): lines urinary tract extending from kidneys to the proximal part of the urethra. ▪ Pseudostratified columnar epithelium: lining in upper respiratory tract where cells are also heavily ciliated. o Secretory epithelia and glands: ▪ Products to be secreted are stored in the cell in vesicles called secretory granules. ▪ Unicellular glands: scattered sensory cells which are common in simple cuboidal, simple columnar and pseudostratified epithelia of many organs. Goblet cell: lining of small intestine and respiratory tract which secretes lubricating mucus that aids in the function of these organs. ▪ Exocrine glands: keep connection with the surface epithelium → ducts ▪ Endocrine glands: lose connection with the original epithelium → no ducts ▪ Connective tissue encloses the gland as its capsule, surround the larger ducts, and forms partitions or septa that separate the gland into lobules. ▪ Some characteristics: Ducts can be simple (ducts not branched) or compound (ducts with 2 or more branches) Secretory portions can be tubular (either short or long and coiled) or acinar (rounded and saclike). Compound glands can have branching ducts and can have multiple tubular, acinar, or tubuloacinar secretory portions. ▪ Epithelial cells in multicellular glands have 3 mechanisms: Merocrine secretion: most common method of protein secretion and involves exocytosis. Holocrine secretion: cells accumulate products as they mature and undergo terminal cell differentiation and then disrupt and release the product into gland’s lumen. (sebaceous glands) Apocrine secretion: products accumulate at cells’ apical ends where they are extruded to release the product together with a bit of cytoplasm and plasma membrane (lipid droplets) ▪ Merocrine products can be further categorized into serious or mucous. Serous cells: stain with basophilic or acidophilic stains. Mucous cells: such as goblet cells and contain mucins which when released from cell, become hydrated and turn into mucus. Some salivary glands are mixed seromucous glands with both serous acini and mucous tubules capped by groups of serous cells. → product is mixture of digestive enzymes and watery mucus. ▪ Myoepithelial cells: found in many exocrine glands are located inside the basal lamina around the basal ends of secretory or duct cells. Their contractions help propel secretory products into and up the duct system. Transport across epithelia: o Na+/K+ pumps. o Transcellular transport o Transcytosis between apical and basolateral membranes in cells. They undergo mitosis and by stem cell populations.